1. Field of the Invention
This invention relates to a method and apparatus for controlling electron current drawn by inactive thruster assemblies in a thruster system. More particularly, this invention relates to using thruster magnetic fields to control the electron current generated by an active cathode assembly of a thruster system and thereby reduces the amount of leakage current drawn by an anode of an inactive thruster assembly.
2. Description of the Art
Thrusters, such as Hall current thrusters and ion thrusters, are an effective mechanism to provide thrust for propulsion and stabilization of planetary or orbital satellites or spacecraft. One conventional way of implementing a thruster system is that each thruster operates from an isolated power supply. In these systems, each power supply is used to provide electrical current to an associated thruster. Since the outputs of the individual power supplies are isolated and can be turned on and off independently there is no problem with current leakage from unused thrusters since no voltage is applied to the unused thruster anodes. This design approach is inefficient since multiple power supplies require additional area and mass on a satellite or spacecraft. Area and mass are limited and, therefore, it is desirable to keep components as small as possible. The conventional implementation of multiple thruster spacecraft propulsion systems does not effectively reduce mass and area.
A conventional thruster system has the anodes of multiple thrusters connected in parallel without isolation switch devices and has the disadvantage that the anode of an inactive thruster draws electron current from active cathode assemblies. This leakage current, drawn by an inactive thruster, drains electron current from the active cathode assemblies and reduces the magnitude of electron current available to active thrusters. This leakage current forces the active cathode assemblies to generate additional electron current to compensate for the losses. The leakage current wastes potentially hundreds of watts of power and can also limit the current available to accelerate ions to produce thrust, thereby degrading system efficiency. It can also make the system totally inoperative since the leakage current can significantly exceed the normal current. This leakage current problem has prevented the direct parallel operation of thrusters in applications where only one thruster is used at a time.
A conventional approach, which attempts to solve the above leakage current problem, is to disconnect the unused thruster anode from the power source using a relay or transistor switch. In order to obtain the desired reliability, such a system may require a plurality of switches for fault tolerant isolation. A drawback to these switches is that they are susceptible to failure, which may prevent an anode from being turned xe2x80x9conxe2x80x9d or turned xe2x80x9coffxe2x80x9d as desired. An uncontrolled anode can cause catastrophic failure of the entire thruster system, which can result in failure of the satellite or spacecraft. The added switch also adds to system cost. This is especially true if the switch must be a redundant configuration of multiple switches.
Some conventional thruster system patents are described as background. U.S. Pat. No. 4,862,032, issued to Kaufman et al. entitled xe2x80x9cEnd-Hall Ion Sourcexe2x80x9d discloses a gas used to produce a plasma that is introduced into a region defined within an ion source. An anode is deposed near one end of that region, and a cathode is located near the other. A potential is impressed between the anode and the cathode to produce electrons which flow generally in a direction from the cathode to the anode. These electrons bombard the gas to create plasma. A magnetic field is established within the region in a manner such that the field strength decreases in the direction from the anode to the cathode. This patent does not disclose utilizing magnetic fields to isolate inactive thruster anodes and thereby reduce leakage current from an active cathode assembly.
U.S. Pat. No. 4,838,021, issued to Beattie entitled xe2x80x9cElectrostatic Ion Thruster with Improved Thrust Modulationxe2x80x9d discloses an ion propulsion system that utilizes an ionizing system for ionizing a gaseous propellant within a chamber to produce a plasma. The ionizing system includes a cathode to provide a source of electrons and anodes to accelerate the electrons to velocities sufficient to ionize the gaseous propellant. An extraction system is used for expelling an ion beam from the plasma. A controller initiates the operation of the thruster by activating the thruster power processor, which in turns activates power supplies. This patent does not disclose using the magnetic field to control electron current and thereby reduce leakage current drawn by an anode that is not producing thrust. U.S. Pat. No. 4,838,021 is hereby incorporated by reference in its entirety herein.
U.S. Pat. No. 5,146,742, issued to lida et al., entitled xe2x80x9cIon Thruster for Interplanetary Space Missionxe2x80x9d discloses an ion thruster operable in an interplanetary space system with plasma generated by microwaves in a propellant atmosphere. A vessel defines first, second and third hollow spaces and a window between the first hollow space and the second and third hollow spaces. This ion thruster system does not disclose controlling an ion beam and reducing the leakage current drawn from a cathode assembly by an inactive anode.
As can be seen from the above discussion, conventional thruster systems are not capable of reliably and efficiently controlling anode activity nor are conventional thruster systems capable of preventing an inactive thruster from drawing leakage current from an active thruster. Therefore, the instant invention provides a simplified control system utilizing magnet fields for reliable control of electron current in inactive thrusters connected to a common power bus, thereby reducing the amount of leakage current drawn by an inactive anode. This reduction in leakage current allows operation of the thruster system without relying on mechanical or electronic switches to disconnect the inactive thrusters since nearly all of the electron current produced by a cathode assembly is available for useful operation of the thruster that provides useful thrust for the satellite or spacecraft. The parasitic leakage current that can, in many cases, prevent proper operation is completely eliminated.
It is an object of the present invention to provide enhanced control of a thruster system. Accordingly, one embodiment is drawn to an apparatus for controlling an electron current including a system power supply and a first cathode assembly coupled to the system power supply for generating an associated electron current. A first thruster produces thrust, and has an associated anode and an associated propellant source. A second cathode assembly is coupled to the system power supply for generating an associated electron current when operating a second thruster. The second thruster produces thrust, and has an associated anode and an associated propellant source. A first magnetic device is associated with the first thruster for generating a first magnetic field and a second magnetic device is associated with the second thruster for generating a second magnetic field. The second magnetic field substantially inhibits the electron current produced by the first cathode assembly from reaching the second anode.
A second embodiment of the present invention is drawn to a method for controlling an electron current in a thruster system comprising the steps of:
generating an electron current in a cathode assembly;
discharging the electron current from the cathode assembly;
attracting a first portion of the electron current to an active thruster;
decoupling propellant flow from at least one inactive thruster;
generating a magnetic field associated with each of the at least one inactive thruster and thereby substantially repelling electron current flow to the at least one inactive thruster.
A third embodiment of the instant invention is drawn to a plasma current controlling apparatus. This apparatus has an anode power supply for supplying power to a thruster system. A cathode assembly is coupled to the anode power supply and receives power from the power supply. The cathode assembly produces an electron current. A plurality of thrusters, each of which has an anode, is coupled to the cathode assembly through the power supply. At least one of the thrusters is active and at least one thruster is inactive. Magnets are used to produce a magnetic field to control the electron current produced by the cathode assembly by presenting an impedance between inactive anodes and the electron current. This impedance repels leakage current drawn by an inactive thruster.